Injectors which are used to inject a small sample into a chromatographic column, have generally been either of two types. One type is an injector with a bypass conduit (bypass loop), such as described in U.S. Pat. No. 3,961,534 by Gundlefinger, wherein a mobile phase liquid (solvent) is always applied under pressure to the column port of the injector, and the sample is applied to the column by connecting a sample chamber (sample loop) to flow in parallel with the solvent. The streams from the bypass loop and sample loop are united before entering the column. Such parallel flow has the advantage of providing an uninterrupted flow into the column, but has the disadvantage that resolution and sensitivity are decreased because of dilution of the sample by the solvent. There is a possibility of major dilution if there is a particle or other partial obstruction in the sample loop.
Injectors are available without bypass loops, such as described in U.S. Pat. No. 4,182,184 by Bakalyar, wherein the solvent initially flows into the column, and this flow is terminated and sample flow into the column begins when a rotor is turned. Such non-bypass injectors have the advantage of avoiding dilution of the sample with solvent. However, they have the disadvantage that the interruption of flow can cause detector baseline noise and reduced column efficiency.
The interruption of flow occurs in non-bypass injectors, during a brief transition period while the rotor is turning between the load and inject positions. At the beginning of this invention period, the flow rate and pressure in the post-injector components (column and detector) drop to zero rapidly. Pressure in the pre-injector components (pump and connecting lines) rises rapidly, since the pump keeps delivering solvent to the dead-ended injector inlet port. When flow is reestablished at the end of the transition period, a pressure surge travels down the post-injector components. These transients cause many detectors to produce baseline disturbances, since they are somewhat flow sensitive. Also, the transients, if they are large enough, cause column efficiency to deteriorate, because the transients change the packing geometry of the stationary phase particles.
The rate of the pre-injector pressure rise during the transition period depends on the flow rate, pre-injector fluid volume, fluid compressibility, and mechanical compliance. The magnitude of the rise depends on the time duration of the flow interruption. Similarly, the magnitude of the pressure shock to the column and detector depends on the duration of the flow interrupt. The period of interruption of prior non-bypass injectors, when used with typical pre and post-injector components and typical flow rates, is large enough to cause measurable loss of efficiency over a period of time. Although any one injection (transient) event usually causes only a small efficiency loss, the cumulative effect of many injections results in a serious shortening of useful column lifetime, due to a continuously degrading efficiency.
An injector which avoided significant dilution of sample with solvent, while also avoiding significant interruptions in flow of liquid into the column, would improve the performance of the chromatographic system without sacrificing column lifetime.